Television receiver contrast and brightness control



May 1, 1951 H. B. s. BRABHAM TELEVISION RECEIVER CONTRAST AND BRIGHTNESS CONTROL Filed Feb. l2, 1948 :reproduced image.

Patented May 1, 1951 TELEVISION RECEIVERv CONTRAST AND BRIGHTN ESS CONTROL Hugh B. S. Brabham, Wembley, England, assignor to Hazeltine Research, Inc., Chicago, Ill., a corporation oir" Illinois Application February 12, 1948, `Serial.No.'7,l'69 In .GreatBritain May 21, 1947 The present rinventionrelates to television receivers and is particularly directed to the improvement `of their contrast-brightness characteristics. The expression contrast-brightness characteristics is lused to mean the correlated eiects of contrast and brightness adjustments in the receiver and, -speciiically to .denote the effects on the brightness of the reproduced image occasioned Vby variations in the contrast.

It .has been found that the eye `.of an vobserver -is responsive primarily tothe contrast or ,ratio between illumination values rather than .to the .absolute valueof any `given shade-of illumination. VFor this reason, it is desirable to provide in a television receiver an arrangement for adjusting the extremes `of the values of illumination inthe Such an adjustment is referred to as ya contrast control and permits adjustment of the receiver in accordance with conditions of reception and the type of picture being received. It usually is in the nature of a gain- .control adjustment. associated with the amplifying stages preceding the reproducing device.

:It .is also desirable to include in the receiver a vbrightness control so that a given signal amplitude corresponding toa particular shade value of Vthe televised image, suchas its black level, is .es- -tablished at a desired point with respect to the signal input-brightness characteristic of thereproducing device. This operatingrequirement is desirable in order that signal components representing other shade 4levels shall appear with the proper relative light values in the reproduced .image. Usually, the brightnesscontrcl takes the form ofa potential divider or the like foi` determiningvan operating bias potential of the reproducing device. It is further desirable tocorrelate the adjustments of the contrast and vbrightness controls so that the various shade'levelsin thereproduced image retain their true relative values with contrastadjustments.

Inconventional.receivers of one type, the television signal as applied to the picture-reproducing device is stabilized with respect to the peaks cf its synchronizing-signal.components and contrast .regulationisachieved by an automatic gain control included in one of the video-frequency amplier stages. Such an arrangement does notpreserve the.contrast-brightness characteristic of the receiver'as uniform as may be desired in certain i installations.. This .is because the black. .level of thetelevision signal varies .relative to the vpeaks of the synchronizing components as the contrast control is adjusted. .As a `consequence,` the black `'level also variesrelative toits desiredponton the 2 claims. (o1. 17a-7.5)

signal input-brightness characteristic of thereproducing device. Accordingly, With such receivers a compensating adjustment of the brightness control is necessary any time the contrast is varied if .the black level is to be maintained lixed with respect to the desired operating point on the signal input-.brightness characteristic of the reproducing device.

In .another prior receiver, the contrast control is in the form of an adjustable voltage divider associated with a video-frequency amplier, while .an adjustable potentiometer coupled 'to the cathode of a cathode-ray reproducing tube functions as a brightness control.V These adjustable .elements are mechanically `interconnected and unicontrolled, whereby the brightness setting is compensated for each variation in the contrast con- 'trol to keep the black level fixed with respect to a given operating point on the signal input-brightness characteristic of the reproducing device. This arrangement performs satisfactorily only when the intensity of the received signal at the detector of the receiver is closely maintained at a xed value. Further, it necessitates the use of unicontrol mechanisms which may be undesirable.

Still other receivers are provided with auxiliary arrangements which stabilize the video signal, prior to application thereof to the image-reproducing deviceJ with respect to an amplitude level which represents black shade value ofthe transmitted image. In such arrangements, black vlevel of the signal is maintained at a iixed point onthe signal input-brightness characteristic of thereproducing device and the contrast may be adjusted without any need for compensating changes in the adjustment of the brightness control. The present invention achieves essentially .the same result but with a circuit .arrangement that is more simplified and less expensive than those of such other prior arrangements. l

It is an object of the present invention, therefore, to provide a television .receiver having an improved arrangement for maintaining a desired contrast-brightness characteristic for the imagereproducing device.

It is another objectvof the invention to provide a television receiver having an arrangement of simplied and inexpensive construction for establishing a desired contrast-brightness characteristic for the image-reproducing device.

It is a further object ci .the invention vto provide a televisionreceiver having Aan'improved and simplifiedbrightnessy control which .is self-compensating for contrastadjustments.

. cillator-modulator I 3.

In accordance with the invention, a television receiver comprises a controllable-gain amplier, including a vacuum tube having an anode electrode, for amplifying na television signal from which video-frequency components may be derived and an adjustable gain control included in the amplier. The receiver further includes a cathode-ray tube having input electrodes coupled to the amplifier and responsive to the video-frequency components and having a given signal input-brightness characteristic for reproducing images in response to the video-frequency components and with a contrast determined by the adjustment of the gain control. A circuit arrangement is provided for applying an operating bias to the input electrodes of the Ycathode-ray tube. This arrangement comprises an impedance, bypassed for video-frequency components of the television signal, for connecting the anode electrode to a unidirectional potential source and for developing a rst component of bias potential having a value varying substantially linearly with adjustments of the gain control. TheA arrangement further comprises a brightness control potential divider, connected in series relation with the impedance across the potential source, for developing an adjustably fixed component of bia-s potential of such value that the net operating bias applied to the cathode-ray tube establishes at a preselected point on its brightnesscharacteristic that amplitude level of the amplified vvideo-frequency components which represents a predetermined shade level ofthe televised image.

For a better understanding of the pre-sent invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In'the drawing, Fig. l is a circuit diagram,-

partly schematic, of a complete television carrier-wave signal receiver embodying the invention and arranged for the translation of positively modulated signals, while Fig. 2 represents a portion of the receiver of Fig. 1 modified to accommodate negatively modulated television signals.

Referring now more particularly to Fig. l, the television carrier-wave signal receiver there represented is of the superheterodyne type and is intended to utilize positively modulated television signals in which increasing amplitude values designate picture components of increasing illumination. This receiver includes a radiofrequency amplifier IG of any desired number of stages having its input circuit coupled to an antenna system II, I2. The output circuit of the radio-frequency amplifier Ill is coupled to an os- Coupled in cascade with the oscillator-modulator I3, in the order named, are an intermediate-,frequency amplifier I4 of one or more stages, a detector I5, and a video-frequency amplifier I6, each of which will be considered in more detail hereinafter. An imagereproducing device I'I of the cathode-,ray tube type is connected to the output circuit of the video-frequency amplier I6. rIhere is also coupled to detector I a synchronizing-signal separator I8. Output circuits of the separator I8 at which separated synchronizing signals appear are coupled to a line-scanning system I9 and a field-scanning system Z. The output circuits of these scanning systems, in turn, are coupled to appropriate beam-deflecting windings associated with the tube I1. An automatic-contrastcontrol (A. C. C.) source 2| is also coupled to the oscillator-modulator I3 and its output circuit is coupled to the input circuit of one or more of the tubes of the radio-frequency amplier IE! and the oscillator-modulator I3 in conventional manner.

A sound-signal translating apparatus 22 is coupled to another output circuit cf the oscillator-modulator i3. It may include one or more stages of intermediate-frequency amplification, a detector, one or more stages of audio-frequency amplication, and a sound-reproducing device.

It will be understood that the various components thus far described, with the exception of the brightness-contrast contral arrangement for the cathode-ray tube I7, may be of any conventional construction and operation. The details of such components are well known in the art rendering a further description thereof unneces- Sary.

Considering brieiiy the operation of the receiver as a whole and neglecting for the moment the speciiic operation of the brightness-contrast control arrangement, a desired modulated carrier-wave television signal is intercepted by the antenna system II, I2. This signal is selected and amplified in the radio-frequency amplifier I0 and is applied to the oscillator-modulator I3 where it is converted to an intermediate-frequency signal. lIhe intermediate-frequency signal is selectively amplied in the amplifier I4 and is detected by the detector I5 to derive the modulation components thereof. These modulation components, which comprise synchronizing components and video-frequency components, are applied to the video-frequency amplier I6 for amplication. Thereafter they are applied to the input circuit of the image-reproducing tube II to modulate the intensity of the electron beam thereof in accordance with the video-frequency components. The synchronizing-signal components of the received signal are separated from the video-frequency components thereof in the separator I8 and are used to synchronize the operation of the line-scanning and held-scanning systems I9 and 20, respectively. These systems generate scanning signals of saw-tooth wave form which are properlyI synchronized with reference to the received signal and are applied to the beam-deflecting windings associated with the image reproducer, thereby to deflect the cathoderay beam of the tube I'I in two directions normal to each other to reproduce the received television image.

The automatic-contrast-ccntrol or A. C. C. signal derived in unit 2| is eiective to control the amplication of one or more or" units IIJ and I3 to maintain the signal input to the intermediatefrequency amplifier I4 Within a relatively narrow range for a wide range of received signal intensities. y

The sound-signal modulated carrier wave accompanying the desired vision-modulated carrier wave is also intercepted by the antenna system i I, I2. It is selected and amplified in the radiofrequency amplifier IG and is applied to the oscillator-modulator I3 where it is converted to a sound-modulated intermediate-frequency signal. The sound-modulated intermediate-frequency signal is applied to the sound-signal translating apparatus 22 wherein it is amplified and detected to derive the modulation components which are .further amplified and reproduced by the soundreproducing device.

Referring now Vmore particularly to the improved arrangement for effecting adjustments of vv| by-passed by a condenserfz.

`hasff'an anode-load impedance 53-.and isz coupled aecomo 5%.; the contrast' and -brightnesseof the image1repro. duced"ik by the receiver, this` arrangementzconr.-

"prisesa controllable-gain carrier-wave ampliiier lfor: amplifying a carrier-wave'television signal. 'Thee expression' carrier-wave"amplert" is `iii-- 'tended to designate an: amplifier for translating' a.lnfidulated"y carrier-Wave television signal prior A'tirfina-'lidetectionf andimay -be either; a radio-fre -uuency or an intermediatefrequency'amplien l "In the pre'sent .embodiment of. the invention, the"'contiastbrightness control arrangement is 'incorporatedin vone stage of. a twof-,stage inter.- mediate-ififequency amplifier Iii although it will cuitfof. ther -oscillator'-modulator I3fthroughz a `transformer' Z5! and'. a. condenserv 21'. The` cathode circuit of the tube:includes.invseriesfselfr-biasf ing resistors. 28 and 29, and-'iisv completed to 'ground through: ai third resistorx, these resis.b

tors being by-passed for intermediate-frequency components by a: condenser 3|. The anode cir'- y cuit of: the tube: 2 Blincludesan: anode-load impedance 321and1 is'- coupled to la unidrectionalspotential source, indicated' `lefB,-thrnugl'rana additional impedance shown as'. a variable .resistor 3'3. 'which is-v decoupled fervideo-f'requency components by a by-.pass condenser 34. A voltage-divider network; `provided by' a iixed4 resistor 35 and a variable resistor ris'connected across the unidirectional; potential source: andl byv ay circuiti connection. betweenither junction of the resistors 35, 3B and the junctionoithe resistors 28iand Z-Sforms with the resistors 29 an'dt30 ofrthe'cathode circuit an. adjustable gain: control included in the rst l stage of the' intermediate-frequency amplifier.

The.' second intermediate-frequency amplifier stage. including a: tetrcdetype-of tube 40,. is essentially Similar: to the one` just described but operatesfwith asubstantiallyxed gain, It has an input circuit coupled to the output circuit of the' preceding stage through a `'condenser 4I and a transformer 42.

The detector l5 is of conventional-construction andincludes `a diode rectiervi and a load resistor vlili; byspassed forintermediate frequencies by a condenser.v 4J. The detector is coupled through a condenser 48.' to theoutput circuit of thelast stageiof the intermediate-frequency'amplifier I4.

. Thevideo-frequency'amplifier I6 is also of conventional construction,v including afpentode type of tube having inputelectrodes Whchfare conductivelyA connected across the load. impedance 46` of the detector l5 through abiasing Vresistor The tube 5.0

tothe; unidirectionalsource +B througha decouplingryresiston 5tv by-passed by-a condenser' 55.

"Thecath'odeof the-reproducing tube I Iyis directly connected with the anode-of the video-frequency 'amplifier 50.

The contrast-brightness characteristicv of the fcathodefray tube i1- is under the controlV of acir- Acuit arrangement for applying tor the input elec- Atrode of the `cathode-ray tubel an operating bias which includes. tvvovv components. The first of quency amplifier The secondcomponent is an ponents' are combined in the input circuit ofthe @mccasey-tese inthe netcperatmgbias ap pliedtotth'e tube estabiishesatra preselectedpoint on. the'v signal; input-bri'ghtn'ess characteristic thereof; that amplitude: level ofi' the amplified television. signal. which? represents a. predeter `mined; shade: level: of the televised image; suchv yas its black level.. Although.; these; twoy components off biaspotential` may be: developed inV circuits which. are 4electa".ica'llyf independent off-one: an-y ment comprisesthe resistor 33irr the ahodecircuit ofthe tubefZ' for` developingthe: component of bias potentialvarying Withadjustmentsof'the gai'rrcontrol of'thattube.. It'furthercompris'es a brightnessecontroll potentiometer 601 for develop.- ingl theradjustablyfixed,component of'bias poten'- tia'lf. The potentiom'feter 60' is; alsov included in the'. iirst stagev of? the`r intermediateffrequency vvamplifier, being-connected in series" relation-*with impedance 33 acrossvthe potentialsource '-l-B. lIhe adjustable tap. ofthe `potentiometer 6U is conductively connected with the brilliancy-control electrode (ilI ofthecathode-ray tubev |:1`and "the low-potential terminal of the potentiometer is' coupled with the cathode ofthe cathode-ray tubethrough tliespace-current path oi'v the videofrequency ampler'SD. I

In considering theI operation of the described arrangement for determining the contrastbrightness characteristic of the receiver, it will beassumedthat the tube-Zioperates on the linear portionv ofu its input-output characteristic Aand that itsfanode' current has a direct-current com? ponent which Varies: substantially linearly with variations in they over-all gain, as nieasured-between the input circuit of the intermediate-frequency'amplier I4 and thecathode-ray tube I'T, effected by adjustments of the gain control' provided by resistors 29, 30 and 36. For any particular setting ofv the contrastecontrol resistor 3S, the brightness-control potentiometer mayl be so adjusted that the net operating bias applied tothe cathode-ray tubel1 establishes that amplitude level-of the television signal' whichrepresents black level of thetelevisedimage at approxf imately cutoff of' the cathode-ray beam. With such an adjustment, the cathode-ray tubey II'fresponds tothe translated television signal and reproduces images with a contrast determined Aby the setting of the contrast-control resistor- 36. Signal' `levels of'Y theA appliedv television signal which corresponds Vto black, or to blacker' than black 'as during4 the synchronizingfsignal inter nvalsbias the cathode-ray tube to anode-current Since the resistor 313 isby'epassed for video-frevduency components, the voltage developed thereacross is a measurer of the direct-current component of' the anode current vofA the'` tube-25.

Therefore, the adjustmentof the contrast control is manifested byL a 'change' ini tha potential developed-across the active lportion ofiV the 'resisthe factor a/b is approximately equal to 0.3.

kto the signal input-brightness characteristicof the cathode-ray tube. In other words, amplitude .levelsof the amplified television signal which represent black shade of the televised image are retained at that point on the brightness characteristic of the cathode-ray tube which corresponds with anode-current cutoi even though `the contrast-control resistor 36 is adjusted to vary the contrast of the reproduced image.

Theoretically, the net bias potential required forthe cathode-ray tube il to maintain the black-level amplitude of the amplifiedl television signal at anode-current cutoiT even with adjustments of the contrast control is realized by satisv.-fying two conditions. The first condition is dened by the following expression:

' R1 1n2 =r.a/b 1) where R1 is the Value of resistance of that part of the resistor 33 traversed by the anode current v of the tube 25; n2 is the ratio of that part of the potentiometer El) between its tap and the junction of the potentiometer and the resistor 33 to the Whole of the potentiometer 6U; r is the slope of the gain-control characteristic of the tube 25 obtained by plotting the change in unidirectional voltage (V) on the cathode of the tube against the mean value of the anode current (I) as effected by adjustment of the contrast-control .resistor 35 for a constant peak-to-peak value of intermediate-frequency signal supplied to the input circuit of the tube 25; and the constante/b is a factor determined by the specifications of the television signal designating the relative amplitude level of that signal which corresponds to black shade level of the televised image. In television systems utilizing positive modulation, the factor a/b is the ratio of the amplitude corresponding to the black level divided by that corresponding to full white. For the case of positive modulation, especially as practiced in England, The operating conditions. defined by Equation 1 are eiective so long as the ratio R1/R2 is negligible compared with n2, wherein R2 is the total value of resistance of the potentiometer 60.-

The second condition to be satisfied is defined asfollows: l

where vc is the value of the potentiai'btweeii the cathode of the cathode-ray tube l1 and the highpotential terminal of the unidirectional source I +B in the absence of an applied television signal;

Vb is fthe value of bias potential required to bias the cathode-ray tube Il to anode-current cutoi; .V is related to the gain-control characteristic discussed in the preceding paragraph and is expressed. by the relation V=V0+1I and VH. T, .is

same as in Fig. 1.

8' lated television signals in which increasing amplitude values denote decreasing shades of illumination." The significant circuit modifications required to convert the receiver of Fig. 1, to one for translating negatively modulated signals are :indicated in Fig. 2 which shows the anode circuits of the amplifier tubes 25,` 40 and 5l]Y as we ll vas the connections to the input electrodes of the vcathode-ray tube Il'. It will be seen that for .this embodiment the brilliancy-control electrode 6l of the cathode-ray tube is-directly connected -to the anode terminal of the video-frequency amplifier 5U while its cathode is connected to the adjustable tap of the potentiometer 60. I Otherwise, the circuits of the intermediate-frequency amplifier i4, the detector I5, the video-frequency amplifier I6, 'and the cathode-raytube ll are the Accordingly corresponding parts of the circuits of Figs. 1 and 2 are identied byl-similar reference Characters. 1

The described arrangements are eiective automatically to maintain the black-level amplitude o ffthe television signal at a preselected point on the signal input-brightness characteristic- `o f -the cathode-ray tube I1 even though. the gain-'controlresistor 36. is adjusted to achieve any desired 'contrast within the operating limitations of the "gain adjustment. For optimum results, it-is preferred that the receiver include an A. C. C. source, as explained above, to maintain the intensity of the' intermediate-frequency signal supplied to amplier I4 at a substantially ixed value. Each arrangement is exceedingly simple in constructionr and establishes a desired contrast-brightness characteristic for the television receiver with -small additional expense.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes anclmodiflcations may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fallwithin the true spirit and scope of the invention.

What is claimed is:

1. A television receiver comprising: a controllable-gain carrier-wave amplifier, including a vacuum tube having an anode electrode, for amplifying a carrier-wave television signal from which video-frequency components maybe derived; an adjustable gain control included in said amplier; a cathode-ray tube, having input electrodes coupled to said amplifier and responsive to said video-frequency components and having a given signal input-brightness characteristic, for reproducing images in response to saidy video-f'requency components and with a contrast determined by the adjustment of said gaicontrol; and a circuit arrangement forfapplying an operating bias to said 'input electrodes 'comprising an impedance, by-passed for videofrequency components of said televisionsignal,

vvfor connecting said anode electrode to 'a unidirectional potential source and for developing a first componentof bias potential having a value varying substantially linearly with adjustments of said gain control and further comprising la brightness control potential divider, connected in series relation with said impedance v.across said `potential source, for developing an adjustably Xed component of bias potential of such value that the net operating bias potential applied to said cathode-ray tube establishes ata preselected point on said brightness characteristic the amplitude level of the amp-lied video-frequency components which represents a predetermined shade level of the televised image.

2. A television receiver comprising: a controllable-gain carrier-wave amplifier, including a vacuum tube having an anode electrode, for amplifying a carrier-wave television signal from which video-frequency components may be derived; an adjustable gain control included in said amplifier; a cathode-ray tube, having input electrodes coupled to said amplifier and responsive to said video-frequency components and having a given signal input-brightness characteristic, for reproducing images in response to said video-frequency components and with a contrast determined by the adjustment of said gain control; and a circuit arrangement for applying an operating bias to said input electrodes comprising an impedance, bil-passed for video-frequency components of said television signal, for connecting said anode electrode t0 a unidirectional potential source and for developing a first component of bias potential havinga value varying substantially linearly with adjustments of said gain control and further comprising a brightnesscontrol potential divider, connected in series relation with said impedance across said potential source and coupled serially therewith between 10 said input electrodes, for developing an adjustably xed component of bias potential of such value that the net operating bias potential applied to said cathode-ray tube establishes at a preselected point on said brightness characteristic the amplitude level of the amplified videofrequency components which represents a predetermined shade level of the televised image.

HUGH B. S. BRABHAM.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,137,263 Bowman-Manifold Nov. 22, 1938 2,204,992 Holmes June 18, 1940 2,255,484 Dome Sept. 9, 1941 2,269,540 Loughren Jan. 13, 1942 2,280,670 Spielman Apr. 21, 1942 2,438,420 Scoles et al Mar. 23, 1948 Y FOREIGN PATENTS Number Country Date 510,715 Great Britain Aug. 4, 1939 845,897 France Sept. 4, 1939 851,411 France Jan. 9, 1940 

